Our prior application referenced above, describes fibrous ceramic insulation materials formed by dispersing a suspension of ceramic fibers over a form-defining surface, drying the fibers to form a mat, solidifying the mat by soaking it with a sol-gel ceramic precursor, and then drying and curing the sol-gel precursor. The present invention is directed to baffles comprising an improved fibrous ceramic.
In NASA Tech Brief (April 1989), p. 88, entitled "Surface Tension Confines Cryogenic Liquid" by Goddard Space Flight Center, there is disclosed a Dewar containing an all-silica, open-cell sponge which is immune to cryogenic temperatures. The average pore size is small enough to ensure the cryogen is prevented from sloshing during orientation, gravitation and/or acceleration changes.
The present invention utilizes glass microballoons and/or diatoms with a fibrous ceramic, which is set after impregnation with a sol-gel. Glass microballoons are usually about 5-200 microns in diameter and are hollow. Various glasses with different wall thicknesses (in the case of hollow spheres) can be used to obtain the desired mechanical strength. In place of, or in combination with microballoons, diatoms may be used. Diatoms are porous silica inner support structures of certain marine and fresh water algae, having typical size ranges of 5-50 microns.
We have found that the rigidized fiber mat made in accordance with our prior application consists of fibers which are randomly tangled, most of which are oriented in the x-y plane (the z-axis being aligned with the thickness of the mat). When bound with the sol-gel binder, the random three dimensional network provides a material, with a porosity of about 90-95%. However, the voids between the ceramic fibers are filled by microballoons and/or diatoms, thus further rigidizing the structure. The mechanical compression strength is increased as the fibers are supported by the microballoons (and/or diatoms). Isotropic structural properties (i.e., similar properties in the x-y, x-z and y-z planes) are also more closely approximated. Typical densities of the fibrous ceramic are in the range of 15-23 lb/ft.sup.3. The useful range may be tailored to be as low as about 7 lb/ft.sup.3. Lower densities may be achieved by substituting lower density microballoons and/or diatoms for higher density binder to achieve an increased strength-to-weight ratio. Selected densities may be achieved for specific applications. The processing time for preparing the ceramic material is substantially reduced, usually by one-half, over the time for making an all fibrous structure without sacrificing strength. Furthermore, problems associated with handling ammonia (used to cure the sol-gel binder) are reduced. In certain applications, the ceramic according to the present invention has a lower dielectric constant than that of the fibrous ceramic of our prior application. For example, at microwave frequencies, ceramic composites according to our prior application have dielectric constants around 1.4, whereas composites of the present invention have dielectric constants around 1.2.
A significant advantage of the improved ceramic is its wickability, especially its wickability to cryogenic fluids, such as liquid oxygen, liquid hydrogen, etc.
The present invention provides internal baffles that are cryogenically compatible with fluids such as liquid fuel for rockets, liquid nitrogen, liquid helium, etc. Such baffles are particularly important to prevent the fuel in an aircraft or space craft from adverse movements which affect the structural integrity and flight characteristics of the craft.
One of the problems with fuel storage in a vehicle is the maintenance of proper readings from electronic sensors regarding fuel level and flow. Particularly in aircraft, the lower the fuel level in a tank, the more critical it is to have accurate information on the remaining amount of fuel. Accordingly, a primary problem is the turbulence at or near the liquid surface, since most fuel level sensors are designed to measure fuel volume as a function of liquid level in a tank. Baffles provided by the present invention may either be permanently attached to the sides of a tank to minimize surface turbulence, or may be moveable to follow the liquid level of the fuel.